A differential GPS receiver is widely used for conducting the precise survey measurements. The differential GPS receiver includes an antenna. The standard GPS antenna is a microwave strip or a patch antenna. Parallelogram-shaped, preferably square, radiating elements are commonly used for patch antennas. In this form, the antenna constitutes essentially a pair of resonant dipoles formed, for example, by two opposite edges of the patch. The microwave patch is of such dimensions that either pair of adjacent sides can serve as halfwave radiators, or the resonant dipole edges may be from a quarter wavelength to a full wavelength long.
The GPS antenna receives the satellite signals from a multiplicity of satellites located virtually anywhere overhead from horizon to horizon. It has been found that the circular polarization of the Rd. Satellite signals is necessary and desirable. Thus, the incoming satellite signal has the right hand circular polarization. Accordingly, the GPS system is also required to have the circular polarization to exclude the dependence of an amplitude of the received signal on azimuth and elevation angle of the incoming satellite signal.
Circular polarization of patch antennas has been achieved in a variety of ways. For example, circular polarization may be obtained when the input coupling point to the signal radiator patch is located within the interior of the patch, along a diagonal line from one corner of the patch to the other. In U.S. Pat. No. 3,921,177 Munson discloses a patch antenna with a feed arrangement that permits the exciting of a pair of orthogonal radiation modes with slightly different frequencies out of phase by 90 degrees. However, the slight variations in the size of the edges of the patch or small variation in the dielectric constant of the substrate can have a significant effect on the resonant frequency and, therefore, on the degree of the circular polarization achieved.
Such shortcomings in microstrip antennas having co-planar radiating elements and feeds have been recognized in U.S. Pat. No. 4,054,874 which discloses reactive coupling of antenna elements. U.S. Pat. No. 4,054,874 issued to Fasset, also discloses capacitively coupled patch antenna elements. However, the bandwidth of the antenna structures so coupled has been found unacceptably narrow.
In U.S. Pat. No. 4,163,236, issued to Kaloi, a corner fed microstrip antenna is disclosed. Kaloi explains how to achieve circular polarization from a single feed line but does not show capacitive coupling to the radiator patch.
Han and Janky, in U.S. Pat. No. 5,165,109, disclose a high performance circularly polarized patch antenna which utilizes a stripline feed circuit to eliminate radiation losses. In one embodiment the apparatus includes a laminated structure having an r.f. radiating conductor affixed on the top side and a feed coupling network within. The r.f. radiating conductor is capacitively coupled to the feed coupling network, a portion of which is sandwiched between suitable ground plane conductors to prevent radiation losses.
The prior art discloses a number of patents on microstrip microwave antennas with circular polarization and broad bandwidth.
In U.S. Pat. No. 5,274,391, Connoly discloses a broadband directional antenna having binary- feed network with microstrip transmission line. The feed network and the dipole antenna utilize impedance matching techniques to provide the most broadband impedance possible.
In U.S. Pat. No. 5,307,075, Huynh describes a monolithically loaded microstrip antenna with a single feed line. The apparatus provides a communication function such as a cellular telephone base station. The antenna includes a ground plane and a group of stacked, planar elements. A director element having a rectangular configuration together with monolithic load tabs is connected to a feed line and spaced above the ground plane. A group of eight of the antennas are positioned in a column to form an antenna array which has substantial vertical polarization, a relatively wide horizontal beam width, and a broad bandwidth.
Iwasaki, in U.S. Pat. No. 5,287,116, discloses an array antenna generating circularly polarized waves with a plurality of microstrip antennas. A microstrip antenna includes a ground conductor plate and a patch opposed to the ground conductor plate with a particular distance, a transmission feed line, and a reception feed line disposed between the ground conductor plate and the patch. Signals are fed from these feed lines to the patch by electromagnetic coupling. The mutual coupling between transmission and reception can be suppressed to a low level, but can not be removed.
In U.S. Pat. No. 5,220,334, Raguenet describes a multifrequncy antenna useable for space telecommunications. The apparatus includes a microstrip patch first antenna operating at one or more frequencies, and a second antenna disposed in front of the first antenna and using the same radiating surface and operating at a different frequency.
Nakahara and Matsunaga in U.S. Pat. No. 5,243,353, disclose a circularly polarized broadband microstrip antenna with a ground plane, a disk-shaped driven element, and a disk-shaped parasitic element. The driven element is located between the ground plane and the parasitic element and is parallel to both of them. The disclosed circularly polarized antenna has the improved impedance bandwidth.
In U.S. Pat. No. 5,319,378, Nalbandian and Lee describe a multi-band microstrip antenna capable of dual-frequency operation. The disclosed antenna can be used in a multi-frequency system without the necessity of having a plurality of separate antennas. The antenna comprises a microstrip having a thin rectangular metal strip that is supported above a conductive ground plane by two dielectric layers which are separated by an air gap or other lower dielectric constant material. Conducting side walls and a rear wall extend between the ground plane and the strip. The ground plane, the strip, the walls and an opening at the front cooperate to form a rectangular resonant cavity. In essence, the cavity is surrounded by conducting surfaces except for the front opening and a small opening in the ground plane that accommodates an antenna feed. The front opening of the cavity functions as an antenna aperture through which the antenna transmits and/or receives energy. The antenna feed is coaxial transmission line that provides a means for coupling the antenna to an external circuit. The spaced dielectric layers and the air gap produces higher-order modes which causes dual frequencies.
In U.S. Pat. No. 5,325,105, Kerbs and Anderson disclose an ultra-broadband TEM double flared exponential horn antenna. The apparatus includes an ultra-broadband transverse electromagnetic TEM exponential antenna in which the radiating or receiving structure includes a feed end. Two TEM horn design embodiments are described and differ only in the launching device by which the radiating structure is fed, which converts an input unbalanced transverse electromagnetic wave into a balanced transverse electromagnetic wave. A first preferred embodiment employs a stripline infinite balun as a launching device, while a second preferred embodiment employs a cavity backed waveguide as a launching device. An input coaxial connector introduces an unbalanced transverse electromagnetic wave into the launching device, either the infinite balun or the cavity backed waveguide.
In U.S. Pat. No. 5,289,196, Gans and Schwartz disclose an improved antenna used for a Doppler radar navigation. The improved antenna satisfies a number of very stringent requirements that are tailored to achieve the precise Doppler overwater measurements. An apparatus includes a space duplexed beamshaped microstrip antenna system including transmit and receive antennas, each of which has two groups of interleaved arrays. The array groups are slanted in opposite directions and each is fed from opposite corners of the antenna so that each group utilizes its entire reduced width aperture to create the required beam contours for two beams. To achieve frequency and temperature compensation, one of the antennas is made up of forward firing arrays and the other of the antennas is made up of backward firing arrays.
Sreenivas in U.S. Pat. No. 5,231,406 discloses a broadband, circular polarization antenna for use on a satellite. In one embodiment, signals are fed to, or received by, an array of electromagnetically coupled patch pairs arranged in sequential rotation by an interconnect network which is coplanar with the coupling patches of the patch pairs. The interconnect network includes phase transmission line means, the lengths of which are preselected to provide the desired phase shifting among the coupling patches. The complexity of the array and the space required are thus reduced. In the preferred embodiment, two such arrays are employed, each having four patch pairs. The two arrays are arranged in sequential rotation to provide normalization of the circularly polarized transmitted or received beam.
U.S. Pat. No. 5,210,542, issued to Pett and Olson, discloses a microstrip patch antenna structure having increased bandwidth and reduced coupling while maintaining low profile capabilities. The structure includes a support member having an isolated recess in which an electromagnetically coupled patch pair of antenna elements is positioned, the upper element being substantially flush with the surface of the support member surrounding the recess. To enhance isolation of the elements, the recess walls and the support surface are preferably electrically conductive and connected to ground.
An apparatus including a planar microstrip Yagi antenna array is disclosed in U.S. Pat. No. 5,220,335 issued to Huang. A directional microstrip antenna includes a driven patch surrounded by an isolated reflector and one or more coplanar directors, all separated from a groundplane on the order of 0.1 wavelength or less to provide endfire beam directivity without requiring power dividers or phase shifters. The antenna may be driven at a feed point a distance from the center of the driven patch in accordance with conventional microstrip antenna design practices for H-plane coupled or horizontally polarized signals. The feed point for E-plane coupled or vertically polarized signals is at a greater distance from the center than the first distance. This feed point is also used for one of the feed signals for circularly polarized signals. The phase shift between signals applied to feed points for circularly polarized signals must be greater than the conventionally required 90.degree. and depends upon the antenna configuration.
In U.S. Pat. No. 5,229,777, Doyle discloses a microstrip antenna for radiating a broad bandwidth of input signals. A pair of identical triangular patches are maintained upon a ground plane, with feed pins being connected to conductive planes of the triangular patches at apexes maintained in juxtaposition to each other. Sides of the conductive planes opposite such apexes are grounded and the radiating slots are formed by the other sides adjacent to the apexes and the ground plane. The input signals to the pair of patches are of equal amplitude, but 180.degree. out of phase. The triangular nature of the patches provides a broad range of signal separation such that the resulting microstrip antenna can accommodate a broad range of input signals and radiate the same.
Mason, Tom and Woo in U.S. Pat. No. 5,272,485, disclose a microstrip antenna with a minimum noise feedpoint used in global positioning system (GPS) receivers. The apparatus includes a diagonally fed electric microstrip RHP antenna having a ceramic substrate, a groundplane on one side of the substrate, a rectangularly-shaped radiator attached to the other side of the substrate, and a wire that passes through the substrate and connects to a point on the radiating electrode that provides the predetermined impedance and a noise figure minimum. The output matching network is used for coupling the active device to an external system, such as a Global Positioning System (GPS) receiver.
The prior art describes different types of circular polarized microstrip antennas. However, the prior art does not disclose a system including a GPS receiver having a symmetrically fed n-point circular polarized microstrip antenna. It is desirable to have a GPS receiver using a symmetrical n-point feed microstrip antenna for receiving circular polarized satellite signals.